If this were an adoption rate, then a rate can certainly drop. Imagine 5% of current users upgrade on Monday and then only 2% upgrade on Tuesday. A rate is not "raw user numbers."

In the case of this article, additional new users on an older version of iOS would cause a drop in the percentage of users on iOS 6. For example, I have not yet upgraded my iPhone 4 to iOS 6. If I start using this app today, I'd increase the number of users, but cause a decrease in the precent of users on iOS 6.